This invention relates, in general, to gettering and, in particular, to non-intrinsic top surface gettering of metallic impurities in a semiconductor substrate.
In general, each wafer fabrication process step is capable of introducing unwanted impurities into a semiconductor substrate. Metallic impurities, such as iron, nickel, and copper have been found to degrade electrical characteristics of semiconductor devices. For example, metal atoms in a lattice of a semiconductor substrate may serve as recombination and/or generation centers; thereby increasing junction leakage currents and lowering bipolar transistor current gains. Often, the metallic impurities are contained within precipitates which may form in a multitude of regions in the semiconductor substrate. A precipitate forming at the silicon-silicon dioxide interface may lower a barrier for electrons to enter the silicon dioxide thereby increasing the leakage current through the silicon dioxide film. A precipitate forming at or near a P-N junction may serve as a generation center for electrons and holes, resulting in a large increase in leakage current in the P-N junction.
One approach to eliminating unwanted metallic impurities is to remove the sources of the impurities from the wafer fabrication process. This approach has proven to be expensive, difficult to maintain, and economically unfeasible for older wafer processing lines. However, a practical, yet relatively inexpensive, technique for removing unwanted metallic impurities is gettering.
Gettering is a process for stably attaching atoms of metallic impurities to trap sites sufficiently remote from active areas on a wafer. The trap sites, commonly referred to as precipitation nuclei, are areas of defects in the crystal structure of a semiconductor wafer. Once attached to a trap site, the metallic impurities are prevented from diffusing through the semiconductor substrate. In other words, the attached metallic impurities are removed, essentially, from the useful regions of the semiconductor substrate. The concentration of metallic impurities in the semiconductor substrate is decreased in the vicinity of a trap site; consequently, a metallic impurity concentration gradient results from the sequestering of metallic impurities by trap sites. As a result of the concentration gradient, metallic impurities from other portions of the semiconductor substrate diffuse towards the trap sites. The diffused metallic impurities become attached to the trap sites. As diffusion and trapping of metallic impurities continues, the concentration of metallic impurities in active areas of the wafer, which are considerably displaced from the trap sites, is reduced appreciably.
The two primary gettering techniques are back-side gettering and intrinsic gettering. In back-side gettering, the unused or back side of a wafer is treated creating trap sites capable of sequestering metallic impurities. Because the trap sites are on a back side of the wafer, the metallic impurities are sequestered away from active areas. Various means for creating trap sites are known, including providing a heavy phosphorus diffusion, argon ion implantation, and deposition of a layer of polysilicon. Drawbacks of these techniques are that the phosphorus dopant may promote autodoping, the trap sites created by argon ion implantation may be eliminated by annealing at high temperatures, and the layer of polysilicon may oxidize at high temperatures. In general, the effectiveness of back-side gettering diminishes with high temperature wafer fabrication process steps.
The technique of intrinsic gettering is based on the precipitation of oxygen in the form of silicon dioxide particles in an inactive bulk area of a semiconductor wafer. The precipitated silicon dioxide particles create regions of high stress which trap metallic impurity atoms in the form of precipitates. Although this technique is useful, it requires strict control of the oxygen content in the semiconductor wafer in concert with specific process steps to form the silicon dioxide precipitates. According to Silicon Processing for the VLSI Era, Volume 1, by S. Wolf and R. N. Tauber, precipitation of oxygen does not occur when the oxygen content is below a lower bound. Moreover, too many oxygen precipitates are formed when the concentration of oxygen exceeds an upper bound. In addition, the size distribution of the silicon dioxide precipitates must be controlled to maximize gettering. Further, back-side and intrinsic gettering techniques tend to be one-dimensional wherein the metallic impurities travel essentially vertically through a semiconductor wafer.
Accordingly, it would be advantageous to have a gettering technique that is economical and capable of fitting into current and future wafer fabrication techniques. Moreover, the gettering technique should be somewhat insensitive to thermal cycling and concentrations of constituents now commonly used for gettering such as, for example, oxygen or phosphorus. Additionally, the gettering technique should be capable of inactivating a large quantity of metallic atoms, commonly referred to as having a large gettering capacity. Finally, the gettering technique should not be limited to one-dimensional movement of the metallic impurity precipitates.